Lead oxide and electrolytic process of forming the same



Dec. 24, 1 940. A. L. CHRISTENSEN LEAD OXIDE AND ELECTROLYTIC PROCESS OFFORMING THE SAME Filed Aug. 4, 1957 EORDQO UK Twig N. my m m 0 ME m m11111 Q WNWMMW M mmmmmm u .Q Ill |lhH H| H W .IIIIIHIIIIIHH. H -m m WH IHWH I m @IWIIIII'IIII'IIIHHJHM mh lm A Md Qhvv ukkg QM\\ \NQ QMRvEIQRNWMH m Patented Dew-24, 1940 'APATENT' OFFICE LEAD OXIDE AND ELECTROLYTICPROCESS OF FORJtflNG THE SAME Arthur L. Christensen,

Perth Amboy, N. J., as-

signor to International Smelting and Refining Company, New Yorlr, N. Y.,a corporation of Montana I Application August 4,

7 Claims.

My invention relates to the production of load oxides, more particularlylead dioxide and litharge of special properties and characteristics, andto an electrolytic process for the recovery of lead in the form ofoxides from ores, concentrates, or

from secondary or other sources. Such secondaries, for example; may bethe slimes resulting from the electrolytic treatment of secondary orscrap copper. I

Such slimes or secondaries, and some ores or ore concentrates,frequently contain a number of various elements. For example, someslimes contain lead, copper, arsenic, iron, nickel, tin, sodium,calcium, antimony, zinc, magnesium and silver. Other sources may containsome or all of the above metals and others in addition. The separationof the various metals, or mixtures of them, presents a number ofdifficulties.

My invention avoids these various difllculties and provides a process inwhich dense deposits of lead dioxide of high purity and capable of readyreduction to litharge are obtained by the electrolytic treatment of anitric acid solution or leaching of the metal containing material.

In my invention a metal bearing material to be treated as, for example,a secondary or slime from the electrolytic treatment of scrap copper,and containing tin oxide, lead sulphate, metallic copper and thesulphates or other compounds of zinc, copper, arsenic, nickel, iron andother metals in small amounts, is first roasted to convert the metalliccopper to an oxide. The roasted material or calcine is then leached withcopper electrolyte to remove a large part of the copper.

' roasted residues are washed with water. The removal of copper and ofsmall amounts of zinc, arsenic, nickel and iron, however, is incomplete.The washed slime is then treated with sodium carbonate in a watersolution to convert the lead sulphate to lead carbonate, and theresulting sod um sulphate is removed by washing. The lead carbonate issoluble in nitric acid and serves as the starting material for myelectrolytic process. Y It is removed from the sulphate, free pulp orslime by treatment with dilute nitric acid or a solution containing alarge proportion of free nitric acid.

The lead nitrate solution resulting from th s leaching also containscopper and other impurities in variable quantities, generally small incom- The.

1937, Serial No.157,237.

'parison with the lead content.

analysis is as follows:

A representative The impure lead nitrate solution is then subjected toelectrolysis whereby the lead is deposited on the anodes as leaddioxide, and the copper is deposited on the cathodes as a metallicsponge. Nitric acid is liberated at the anode proportionally to thedeposition of copper at the cathode, and of lead dioxide at the anode,the formation of the lead dioxide at the anode being due to theoxidizing conditions at the anode. As a result of the electrolyticaction, therefore, the electrolyte solution increases in nitric acidcontent and decreases in lead and copper, these two metals, however,being separately deposited. The nitric acid may, therefore, again beused to leach a fresh quantity of the treated secondaries or sourcematerial. The other impurities remain unchanged in the solution, beingtoo small in quantity and so located in the electro-motive series as notto be deposited. If, on long continued use or circulation of theelectrolyte solution they increase to a troublesome point, they may beremoved by evaporation and crystallization. 40

In the continuous operation of my process, therefore, the electrolyteliquid, after being subjected to electrolysis suiliciently to decreasethe lead and copper content and regenerate the nitric acid, is againpassed through the leaching process, coming into contact with freshquantities of the calcined material that has been treated with sodiumcarbonate. Having dissolved additional quantities of lead and copper, itthen returns to the electrolytic bath for a new electrolysis. The

greatly increased. A typical example of such a residue is one of thefollowing composition:

Per cent Lead 2.6- Tin 65.0 Copper 0.2 Antimony 3.0

In carrying on the electrolysis the best results are obtained when thefree nitric acid content is between 30 and 60 grams per liter, the leadcontent between 35 and 50 grams per liter, and the copper contentbetween 2 and 15 grams per liter. In operating the process continuouslyleach liquor is supplied continuously to the electrolytic tanks freenitric acid:

Grams per liter Lead 87.5 Copper 6.7 Free nitric acid 9.4

Inasmuch as. this liquor is admixed with liquor undergoing electrolysis,the concentrations of lead and copper drop and the percentage of freenitric acid increases. Analysis of a typical electrolytic bath solutionshows the following content:

Grams per liter Lead 50.0 Copper 2.5 Free nitric acid 31.0

The overflow liquor is weaker in lead and copper and stronger in freenitric acid than the solution in the bath. A typical analysis shows thefollowing content:

Grams per liter Lead 40.2 Copper 2.1 Free nitric arid 41.9 Ammonia 0.59

It will be understood, therefore, that the fresh leach liquor maycontain larger quantities of lead and smaller quantities of nitric acidthan those best suited for the electrolysis inasmuch as they willbe-immediately mixed with electrolytic liquors of such concentration asto bring them within the desired limits.

During electrolysis the electrolyte liquor is agitated, eithermechanically or by air, to prevent stratiiication and to obtain uniformdeposition of lead dioxide.

In the operation of the electrolytic bath the maximum current density isdesirable for the purpose of obtaining as high a production or capacityas possible. Current densities up to amperes per square foot may beemployed without obtaining too rough a deposition of lead dioxide or ofbulky nodules. These current densities also permit a good retention ofthe copper sponge at the cathode.

The deposition of lead dioxide is also most successfully attained whenan anode containing silicon is employed. An iron silicon alloy isespecially useful as an anode material.

Duriron,

which is an alloy of iron and silicon with other elements in lesseramounts, forms an excellent anode material. Chilex, having the followingpercentage composition- Per cent Copper 60 Silicon 23 Iron 10 Tin 3 Lead2 Manganese 2 also is a desirable anode'material. In general,

however, any silicon alloy having sumcient electrical conductivity maybe used as an anode material.

In order to enable the lead dioxide deposit to be readily stripped fromthe anode, the surface of the latter may be finished to facilitatestripping. A surface prepared by sand blasting is particularly suitedfor this purpose. not altered by continued use. In case a deposit of aminute film of lead dioxide a few thousandths of an inch thick mayremain on the anode and cause a poor retention of the new deposit ifimmediately used, this residual film of lead dioxide may be easilyremoved by placing the anodes and cathodes in the electrolytic cell intheir normal operating positions and short circuiting the cell. The cellthen acts as a battery, causing the resolution of the lead dioxide fromthe anode and a re-solution of some copper from the cathode. After aperiod of time, 24 hours for example, the anodes are entirely clean oflead dioxide and the electrolytic operation may be resumed by connectingthe cell back into its regular circuit.

may be employed. A convenient shape is one in which the anode is longand narrow as, for example, 5 inches wide and 1 inch thick, so that itmay be hung from one end. Four or five such anode bars may be suspendedon a single rod and closely spaced'so that about 100 may be placed in a.single electrolytic cell. This makes a cell of convenient size.

Any suitable material may be used for the cathode as, for example,copper, stainless steels or other metals. resistant to nitric acid. Theuse of any particular metal does not have any influence on the cathodedeposit.

It is desirable to have as large a cathode area as is practicallypossible because this gives an opportunity to lessen the current densityon the cathode, resulting in a better retention of the deposited copperand permitting the use of a higher concentration of copper in theelectrolyte whiich results in less cathodic reduction of nitric aciApparatus suitable for operating the process is shown diagrammaticallyin the accompanying drawing.

In this diagrammatic apparatus, a quantity of lead carbonate or oxideprepared as described above is placed into a leaching vat I into which astream of regenerated nitric acid is circulated through a pipe 2. Thenitric acid, after dissolving the lead and copper and smaller quantitiesof impurities, overflows from the leaching tank I through an overflowpipe 3 into a storage tank 4. From the storage tank 4 the solutionpasses through a control valve 5 into the electrolytic tanks 6 intowhich dip alternate anodes I and cathodes 8 connected, respectively, toa positive main 9 and a negative main ID. The liquor that has beenimpoverished in lead and copper, and

Such a surface is Any suitable shape and arrangement of anodes Thesemetals do not need to be I made.

enriched in nitric acid in the electrolytic tank 6, then overflowsthrough the pipe H into a regenerated nitric acid tank l2, from which itis pumped nto the pipe 2 to return to the leaching tank I. In a typicalinstallation a cell was arranged to take 1212 amperes with a currentdensity of 13 amperes per square foot on the anode. The temperature ofthe cell was about .80? F., although it may vary from this temperature,and the voltage was 2.2 volts. A current efliciency of 92.5% wasreached. The electrolysis could be run somewhat over six days before itwas necessary to strip the deposit. The rate of deposit per run was 1599pounds, and the rate per day was 262 pounds. The deposit of lead dioxidewas vitreous in character and could be easily removed from the anode andbroken down to any desired size. It could be pulverized. An analysis ofthe typical deposit showed it to have the following composition:

Percent Total lead 85.6

Lead dioxide 962 Copper 0.005

Silver -None Bismuth 0.00005 Insoluble 0.06

Water soluble 0.10

Nitric acid 0.008.

Moisture 0.10

It was easily reduced with coke, coal or charcoal, producing a leadhaving the following impurities:

Percent Copper 0.006

The lead dioxide can be easily converted to.

litharge by direct roasting. An acid soluble lead compound is thusobtained of such high purity that it can be used for the preparation ofany lead compound without further purification.

The lead dioxide produced by my process is denser and more resistant todeterioration in storage than lead dioxide heretofore known. It convertsinto a litharge having a coarser crystalline structure and weighing moreper unit of volume, and freer from dust, than litharge made from leadsulphide. Whereas litharge made from lead sulphide was cast into blocksand ground,- the litharge of my process may be made by mere roasting,whereupon the lead dioxide disintegrates upon forming the litharge. Thelitharge obtained by my process is of great purity owing to the purityof the lead dioxide from which it is What I claim is:

l. A process for the electrolytic production of containing silicon.

lead dioxide, which comprises subjecting to electrolysis an aqueoussolution containing between and 50 grams per liter of lead andmaintaining between about 9 and 42 grams of free nitric acid per'liter.

2. A process for preparing lead dioxide electrolytically, whichcomprises electrolyzing an aqueous solution containing between 35 and 50grams per liter of lead, and maintaining between 30 and 60 grams perliter of nitric acid in said solution duringelectrolysis.

3. A process for preparing lead dioxide electrolytically, whichcomprises electrolyzing an aqueous solution containing between 35 and 50grams per liter of lead, and maintaining between 30 and 60 grams perliter of free nitric acid, and between 2 and 15 grams per liter ofcopper in said solution during electrolysis.

4. The process of producing lead dioxide and copper from roastedmaterials containing lead and copper compounds soluble in nitric acid,which comprises leaching said roasted materials with nitric acidcontaining aqueous solution to obtain a leach liquor containing lead inexcess of 35 grams per liter, copper in excess of 2 grams per liter, andfree nitric acid, electrolyzing the solution containing lead, copper andfree nitric acid to deposit lead dioxide at the anode and copper at thecathode, and passing the leach liquors into said electrolytic bath,overflowing the exhausted liquor from said electrolytic bath in suchrelation to the current consumption as to maintain a concentration ofbetween 35 and 50 grams per liter of lead, between 30 and 60 grams perliter of free nitric acid, and between 2 and 15 grams per liter ofcopper in said bath, and leaching the roasted lead and copper containingmaterials with the overflow liquor from said electrolytic bath.

5. A process of preparing lead dioxide from materials containing leadand copper compounds soluble in nitric acid which comprises leachingsuch materials with an aqueous solution containing nitric acid todissolve said lead and copper compounds to form nitrates whilemaintaining the nitric acid content above about 9 grams per liter,passing the resulting solution into an electrolytic bath andelectrolytically depositing lead dioxide at the anode and copper at thecathode and regenerating the nitric acid from the nitrates of lead andcopper and thereafter returning the solution from said electrolysis tosaid leaching of said material. I

6. The process of claim 5 in which the lead is deposited on an anodesurface containing silicon.

7. The process of claim 6 in which said oxide is deposited fromelectrolysis on an anode surface ARTHUR L. CHRIS'I'ENSEN.

